In an optical disk medium using a land/groove recording method, which is typified by a DVD-RAM (Digital Versatile Disk Random Access Memory), signals are recorded on both a recessed recording track formed by a guide groove and a projecting recording track formed between the guide grooves, and further each of the recording tracks is composed of the integer number of recording sectors each having the same length. Further, an address signal portion including information such as track address is added to a head portion of each of the recording sectors. For this type of optical disk medium, a position of an address signal is detected, thereby performing processing such as determining a tracking polarity.
An example thereof is disclosed in Japanese Patent Registration No. 3073744 (Pages 11 and 12, and FIG. 11).
Hereinafter, a prior art address signal position detection apparatus disclosed in the above-described prior art document will be described.
FIG. 11 is a diagram illustrating a track layout of this type of optical disk medium using the land/groove recording method. FIG. 11 shows a disposition of tracks and recording sectors in one zone and a construction of the recording sectors. As shown in FIG. 11, a track of a groove portion (hereinafter, also referred to as a groove or a recessed portion) shown as black in FIG. 11 (hereinafter, referred to as a groove track), and a track of a portion between grooves (hereinafter, also referred to as a land or a projecting portion) shown as white in FIG. 11 (hereinafter, referred to as a land track) are alternately connected once for every track on the disk, thereby forming a spiral recording track. Further, a recording track is composed of the integer number of recording sectors, and a preformatted address signal portion is added to a head portion of each of the sectors. The preformatted address signal portion is recorded when the optical disk recording medium is manufactured. The address signal portion is composed of two parts of a forward part and a rearward part with respect to the scanning direction. The forward part is displaced from the groove portion onto the outer circumference side on the disk medium by half the groove width and disposed, and the rearward part is displaced from the groove portion onto the inner circumference side on the disk medium by half the groove width and disposed.
An address of a groove portion is added to a forward part of an address signal portion of the groove portion, which is disposed in the address signal portion just before the information recording portion thereof and displaced from the center of the groove portion onto the outer circumference side by half the groove width. On the other hand, an address of a portion between grooves is added to a rearward part of an address signal portion, which is disposed in the address signal portion just before an information recording portion of a recording track of a groove portion on the outer circumference side of the recording track of the portion between grooves by one track and displaced from the center of the groove portion onto the inner circumference side by half the groove width. Consequently, the address of the portion between grooves is added to a rearward part of an address signal portion of a groove portion, which is disposed in an address signal portion just before the information recording portion thereof and displaced from the center of the portion between grooves onto the outer circumference side by half the groove width.
Next, the address signal portions for connecting parts each connecting a land and a groove for every track on the disk, and which parts are aligned in the radial direction of the disk, will be described.
In a recording sector just after a connection point between a recording track of a groove portion and a recording track of a portion between grooves, as the disposition of the address signal in the address signal portion thereof, a forward part is displaced from the groove portion onto the outer circumference side by half the groove width and disposed, just as with the disposition of address signals except in the boundaries. A rearward part is displaced from the groove portion onto the inner circumference side by half the groove width and disposed. An address of the groove portion is added to the forward part of the address signal portion, which is displaced from the groove portion just before the information recording portion thereof onto the outer circumference side by half the groove width and disposed, just address values are added except in the boundaries. On the other hand, an address of the portion between grooves is added to the rearward part of the address signal portion, which is displaced from the portion between grooves just before the information recording portion thereof onto the outer circumference side by half the groove width and disposed.
As described above, a first part which is a part of an address signal is displaced from the center of the groove portion into one radial direction, for example, onto the outer circumference side, by a given distance, and disposed, while a second part which is the other part of the address signal is displaced from the center of the groove portion into the other radial direction, that is, such as onto the inner circumference side, by the same distance as the give distance, and disposed, and when the disk is played back, a tracking error signal, that is, a difference signal of a tracking sensor in the radial direction (in the direction of radius) is binarized by two comparators each having a different threshold value from each other to detect the change. This enables tracking polarities of the respective recording sectors to be determined, thereby detecting a connecting point between a land track and a groove track.
The method for detecting the address signal position is disclosed in the above-described prior art document, and FIG. 12 shows a timing chart for the detection disclosed in the prior art document.
The disposition of grooves and preformatted address signals is shown as the signal disposition on the disk surface in FIG. 12. The first half part of the address signal of the groove portion is displaced from the center of the groove onto the outer circumference side by half the track pitch and disposed, while the latter half part is displaced from the center of the groove onto the inner circumference side by half the track pitch and disposed. Thereby, the address signal dispositions are different between a boundary sector portion at which a land track and a groove track are connected and another normal sector portion as is shown by the signal disposition on the disk surface in FIG. 12. Here, a difference between output signals from photoreceptors of a pickup is taken, thereby obtaining a difference signal used for push-pull tracking servo system like the difference signal shown in FIG. 12.
Then, two levels of threshold values, Lth and Rth, are provided for the comparators respectively so as to detect that prepits are displaced into the left and the right with respect to the direction of travel of a light beam by half the track pitch in the address signal portion, respectively. Thereby, a binarized signal L0 indicating that a tracking of the light beam is displaced into the left side (the inner circumference side) with respect to the tracing direction shown in FIG. 12 and a binarized signal R0 indicating that the tracking of the light beam is displaced into the right side (the outer circumference side) are generated. When the difference signal level is Lth or more, L0 is Hi, while when the difference signal level is Lth or less, L0 is Lo. On the other hand, when the difference signal level is Rth or less, R0 is Hi, while when the difference signal level is Rth or more, R0 is Lo. The states of L0 and R0 are as shown by L0 and R0 in FIG. 12.
As for the address signal, since the groove is modulated by the information and discontinued, and takes a form of pit string, the two binarized difference signals L0 and R0 from the difference signal waveform generation unit (not shown) also have waveforms which are modulated by data signal frequency. Then, each of the pit string waveforms is corrected so as to become a waveform of one consecutive pulse in the first half part and the latter half part of the address signal, with using such as a retriggerable mono multivibrator, in order to be able to detect the presence or absence of the first half part and the latter half part of the address signal on the basis of the two input binarized difference signals. L0 is corrected to generate a binarized corrected difference signal L1 and R0 is corrected to generate a binarized corrected difference signal R1. The states of these corrected difference signals L1 and R1 are as shown by L1 and R1 in FIG. 12, respectively.
However, when the prior art address signal position detection apparatus is constructed as described above, there is a problem that when a DC symmetry is imperfect in the difference signal used for the detection between the forward part and the rearward part of the address information portion, a stable detection of address position cannot be made.
FIG. 13 is a diagram illustrating an address position detection signal in the case of the DC symmetry being imperfect between the forward part and the rearward part of the address information portion. A reflected light of a light spot obtained by applying a light onto a track of an optical disk medium is received, thereby obtaining output signals s100, s101, s102, and s103 shown in FIG. 13. The output signals are obtained with using a photodetector 10 comprising photoreceptors 10a, 10b, 10c, and 10d which are divided into four so as to form a cross-in-square shape as shown by signals on the disk surface in FIG. 13. The prior art apparatus is constructed so as to obtain a difference signal used for detecting an address position by subtracting the sum of outputs from the photoreceptors 10b and 10c in the photodetector 10 from the sum of outputs from the photoreceptors 10a and 10d in the photodetector 10. The difference signal corresponding to the first half part of the address information portion has a DC offset smaller than the difference signal corresponding to the latter half part and the DC symmetry is imperfect between the first half part and the latter half part of the address information portion as shown by the waveform of the difference signal in FIG. 13.
The symmetry of the difference signal becomes imperfect because a deviation of an optical axis with respect to a light reflected on the signal surface of the optical disk medium and the like occur due to such as variations in pickups at the manufacturing, and thereby offsets occur in the output signals s100, s101, s102, and s103 from the photoreceptors 10a, 10b, 10c, and 10d. 
Accordingly, when the address position is attempted to be detected using the difference signal and the threshold values Lth and Rth, the difference signal does not exceed the level of the threshold value Lth as shown by L1 and R1 in FIG. 13. Therefore, only one of the address position signals can be detected. When only one of the address position signals can be detected, switching between tracking polarities cannot be performed at the boundary between the land and the groove, and thereby a stable tracking servo cannot be performed. When the setting value of the threshold value is changed so as to be shifted closer to the center value in order to avoid the state where only one of the address position signals can be detected, the detection becomes possible. However, when the setting value is shifted too closer to the center value, the fluctuation amount of the difference signal which is generated in the data recording portion exceeds the threshold value when there is a vibration at the tracking, and a data signal is erroneously detected as an address position signal in the data recording portion. Therefore, switching between tracking polarities is performed except at the boundary between the land and the groove, and thereby the tracking servo cannot be stably performed.
As described above, there is a problem that a prior art address signal position detection apparatus cannot stably detect an address position when a DC symmetry is imperfect.